Voltage controlled oscillator

ABSTRACT

A cascaded voltage controlled oscillator is described that includes a first oscillator stage having a first oscillator stage first input, a first oscillator stage second input and a first oscillator stage output. A second oscillator stage includes a second oscillator stage input and a second oscillator stage output wherein the first oscillator stage output is input to the second oscillator stage input and wherein the second oscillator stage output is fed back to the first oscillator stage second input. A third oscillator stage includes a third oscillator stage input and a third oscillator stage output wherein the second oscillator stage output is fed to the third oscillator stage input.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending U.S. patent application Ser.No. 10/004,521 entitled PHASE SYNCHRONOUS MULTIPLE TANK OSCILLATOR filedconcurrently herewith, which is incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

The present invention relates generally to Voltage controlledoscillators (VCO's). A VCO with enhanced Q and better signal to noiseration (SNR) is disclosed.

BACKGROUND OF THE INVENTION

Voltage Controlled Oscillators (VCO's) are important components inwireless communication systems. FIG. 1 is a block diagram illustrating aconventional differential cascaded VCO with an LC tank. The output ofstage 102 is input to stage 104 and the output of stage 104 is input tostage 106. The output of stage 106 is negatively fed forward to theinput of stage 102 with the positive output being fed into the negativeinput and the negative output being fed into the positive input. Thecascaded stages produce a filtering effect with somewhat improved phasenoise, but the circuit tends to consume a large amount of current.

It would be desirable if an improved cascaded VCO could be developedwith low phase noise so that the signal to noise ratio in the outputsignal can be reduced. Furthermore, it would be desirable if theamplitude could be increased without increasing the current drawn by thedevice.

SUMMARY OF THE INVENTION

An improved cascaded VCO is disclosed. In one embodiment, eachoscillator stage receives an additional input from a second stage inaddition to the conventional input received from a previous stage. Theinputs are selected with a phase relationship that causes the totalinput signal power to each stage to be increased.

It should be appreciated that the present invention can be implementedin numerous ways, including as a process, an apparatus, a system, adevice, a method, or a computer readable medium such as a computerreadable storage medium or a computer network wherein programinstructions are sent over optical or electronic communication links.Several inventive embodiments of the present invention are describedbelow.

In one embodiment, a cascaded voltage controlled oscillator includes afirst oscillator stage having a first oscillator stage first input, afirst oscillator stage second input and a first oscillator stage output.A second oscillator stage includes a second oscillator stage input and asecond oscillator stage output wherein the first oscillator stage outputis input to the second oscillator stage input and wherein the secondoscillator stage output is fed back to the first oscillator stage secondinput. A third oscillator stage includes a third oscillator stage inputand a third oscillator stage output wherein the second oscillator stageoutput is fed to the third oscillator stage input.

These and other features and advantages of the present invention will bepresented in more detail in the following detailed description and theaccompanying figures which illustrate by way of example the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 is a block diagram illustrating a conventional differentialcascaded VCO with an LC tank.

FIG. 2 is a block diagram illustrating a multiple input cascaded LC tankVCO.

FIG. 3 is a block diagram illustrating a two input LC tank oscillatorused in one embodiment.

FIG. 4 is a block diagram illustrating a cascaded LC tank oscillatorwherein a second input is obtained for each stage by feeding forward theinput from a previous stage.

FIG. 5 is a block diagram illustrating an improved oscillator stage foruse in the cascaded LC tank oscillator shown in FIG. 4.

DETAILED DESCRIPTION

A detailed description of a preferred embodiment of the invention isprovided below. While the invention is described in conjunction withthat preferred embodiment, it should be understood that the invention isnot limited to any one embodiment. On the contrary, the scope of theinvention is limited only by the appended claims and the inventionencompasses numerous alternatives, modifications and equivalents. Forthe purpose of example, numerous specific details are set forth in thefollowing description in order to provide a thorough understanding ofthe present invention. The present invention may be practiced accordingto the claims without some or all of these specific details. For thepurpose of clarity, technical material that is known in the technicalfields related to the invention has not been described in detail so thatthe present invention is not unnecessarily obscured.

FIG. 2 is a block diagram illustrating a multiple input cascaded LC tankVCO. LC tank oscillators are shown in the illustrated embodiment and arereferred to extensively herein. In other embodiments, other types ofoscillators are used and are connected in a similar manner as shown. Inaddition, it should be noted that each LC tank oscillator may be singleended. In the example shown, LC tank oscillators with two inputs areshown.

Oscillator stage 204 receives input from stage 202 in a manner similarto that described above. In addition, oscillator stage 204 also receivesas an additional input the output signal from oscillator stage 206 whichis positively fed back to oscillator stage 204. Similarly, oscillatorstage 206 receives input fed forward from stage 204 and fed backpositively from stage 208. The output of stage 208 is fed forwardnegatively to stage 202 with the positive output being input to thenegative input and the negative output being input to the positiveinput. Stage 202 also receives input from the output of stage 204, whichis fed back positively. The 45 degree phase difference between the twoinput signals to each stage of the four stage amplifier causes the inputpower to be increased and enhances the amplitude of the output. Anadvantage of this phase arrangement is that the 45 degree added signaltends grow faster than the orthogonally added noise that is introduced,which enhances the SNR.

In other embodiments, different arrangements of the input and outputconnections are used. In some embodiments, the second input is obtainedfrom a stage that is not immediately next to the stage that is receivingthe input, i.e. stages may be skipped. In addition, different numbers ofstages are used in other embodiments. The phase relationship among theinputs is arranged so that the input power is increased.

FIG. 3 is a block diagram illustrating a two input LC tank oscillatorused in one embodiment. Oscillator 300 includes positive input nodes 302and 304 and negative input nodes 312 and 314. The output is taken atoutput nodes 322 and 324. Advantageously, NMOS transistors are used atthe input nodes to increase ramp up speed.

FIG. 4 is a block diagram illustrating a cascaded LC tank oscillatorwherein a second input is obtained for each stage by feeding forward theinput from a previous stage. Then inputs for stage 404 are obtained fromthe output of stage 402 and the fed forward input of stage 402. Theinputs for stages 406 and 408 are obtained in similar fashion and theinputs for stage 402 are obtained by negatively feeding forward theoutput of stage 408 and also negatively feeding forward the input fromstage 408. The feed forward relationship increases the ramp up rate ofthe signal.

FIG. 5 is a block diagram illustrating an improved oscillator stage foruse in the cascaded LC tank oscillator shown in FIG. 4. Oscillator stage500 includes positive input nodes 502 and 504 and negative input nodes512 and 514. The input from the immediately previous stage is appliedacross nodes 502 and 512 and the input from one stage before theimmediately previous stage is applied across nodes 504 and 514. Acombination of NMOS and PMOS devices are used. In general, PMOS devicestend be slower than NMOS. In the design shown, the PMOS devices receivethe signal before the NMOS devices and can be slower. Therefore, smallersize PMOS devices (as small as the NMOS devices) can be used. Thus,power consumption is reduced.

An improved cascaded VCO has been described. Each oscillator stagereceives an additional input from a second stage in addition to theconventional input received from a previous stage. The inputs areselected with a phase relationship that causes the total input signalpower to each stage to be increased. In addition, improved LC tankoscillators have been disclosed for use in such cascaded VCOarchitectures.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing both the process and apparatus of the present invention.Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalents of the appended claims.

What is claimed is:
 1. A cascaded voltage controlled oscillatorcomprising: a first oscillator stage having a first oscillator stagefirst input, a first oscillator stage second input and a firstoscillator stage output; a second oscillator stage having a secondoscillator stage input and a second oscillator stage output wherein thefirst oscillator stage output is input to the second oscillator stageinput and wherein the second oscillator stage output is fed back to thefirst oscillator stage second input; a third oscillator stage having athird oscillator stage input and a third oscillator stage output whereinthe second oscillator stage output is fed to the third oscillator stageinput; and a fourth oscillator stage having a fourth oscillator stageinput and a fourth oscillator output; wherein: the oscillator stages areLC tank oscillators; the third oscillator stage output is fed to thefourth oscillator stage input; the first oscillator stage first inputincludes a first oscillator stage first positive input and a firstoscillator state first negative input; the fourth oscillator stageoutput includes a fourth oscillator stage positive output and a forthoscillator stage negative output; and the fourth oscillator stagepositive output is fed to the first oscillator stage first negativeinput and the fourth oscillator stage negative output is fed to thefirst oscillator stage first positive input.
 2. A cascaded voltagecontrolled oscillator comprising: a first oscillator stage having afirst oscillator stage first input, a first oscillator stage secondinput and a first oscillator stage output; a second oscillator stagehaving a second oscillator stage input and a second oscillator stageoutput wherein the first oscillator stage output is input to the secondoscillator stage input and wherein the second oscillator stage output isfed back to the first oscillator stage second input; a third oscillatorstage having a third oscillator stage input and a third oscillator stageoutput wherein the second oscillator stage output is fed to the thirdoscillator stage input; wherein the phase relationship of signals beingfed to the first oscillator stage first input and the first oscillatorstage second input is such that the total input to the first oscillatorstage is enhanced and the phase difference between the signal being fedto the first oscillator stage first input and the first oscillator stagesecond input is approximately 45°.
 3. A cascaded voltage controlledoscillator as recited in claim 2 wherein the oscillator stages are LCtank oscillators.
 4. A cascaded voltage controlled oscillator as recitedin claim 2 wherein there are four oscillator stages.
 5. A cascadedvoltage controlled oscillator as recited in claim 2 wherein theoscillator stages are single ended.